Season’s first smog episode has hit Delhi-NCR, says new CSE assessment

Smog has also wrapped the entire Indo-Gangetic Plain  

  • The duration of the ongoing smog episode is expected to be longer than seven days.
  • While unfavourable weather conditions, farm fires and Diwali crackers have triggered the first episode, the contribution of farm fires has been the lowest in the last four years.
  • Deeper analysis dispels the myth that winter pollution is only a particulate problem -- it is also a noxious cocktail of toxic gases including ozone, carbon monoxide, nitrogen dioxide and sulphur dioxide.
  • 5 concentration on Diwali night (8 PM-8 AM) has been most intense since 2017, while the ratio of sulphur dioxide and nitrogen oxide has been high, indicating the impact of firecrackers.
  • The ongoing severe smog episode requires urgent emergency measures across all sectors to prevent further trapping of pollution and worsening of the situation, while a longer term strategy needs to be put in place right away. 

See the associated graphs of this CSE analysis click here

New Delhi, November 10, 2021: The first severe smog episode has hit Delhi and the National Capital Region (NCR) and is expected to last for another two days. The smog has engulfed the entire Indo-Gangetic Plain, shows a new analysis by Centre for Science and Environment (CSE). The region is in the grip of a public health emergency and requires urgent and drastic measure to shave off the pollution peak. 

“Predictably, the season’s first episode has been triggered by the combined effect of unfavourable weather conditions (cool and calm winds and inversion), stubble burning and firecrackers. But even though the seasonal smog in Delhi is intense, the average daily contribution of smoke from farm stubble fires from the middle of October to November 8, 2021 has been the lowest in last four years. But since November 6, its contribution has peaked to reach 48 per cent on November 7, and it is still elevated,” says Anumita Roychowdhury, executive director, research and advocacy, CSE.   

“While very high concentrations of PM2.5 have hogged attention, the levels of gases – ozone, CO or NO2 -- have remained elevated during this smog episode. Also, the ratio of SO2 to NO2 increased on Diwali night indicating increased pollution load from firecrackers. The PM2.5 concentration on Diwali night (8 PM to 8 AM) has been the most intense since 2017,” says Avikal Somvanshi, programme manager, Urban Lab, CSE.   

This analysis is based on the real time data available from the current working air quality monitoring stations in Delhi-NCR and the larger Indo-Gangetic Plain. For this analysis, a huge volume of data points have been cleaned and data gaps addressed, based on the USEPA methodology. This analysis covers 156 continuous ambient air quality monitoring stations (CAAQMS) spread across 67 cities in Punjab, Chandigarh, Haryana, Delhi, Rajasthan, Uttar Pradesh, Bihar and West Bengal. Meteorological data for the analysis has been sourced from the Palam weather station of India Meteorological Department (IMD). Fire count data has been sourced from NASA’s Fire Information for Resource Management System, specifically, the Visible Infrared Imaging Radiometer Suite (VIIRS) product has been used. Estimates of contribution of farm stubble fire smoke to Delhi’s air quality has been sourced from Ministry of Earth Science’s System of Air Quality and Weather Forecasting and Research (SAFAR). 

Key highlights 

Season’s first smog: PM2.5 concentration in Delhi skyrocketed beyond 250 microgramme per cubic metre (ug/m3) (breakpoint for severe AQI category) on Nov 4th and is still hovering over it seven days later. It hit its peak concentration of 501 ug/m3 on 5th November, then gradually receded to 256 ug/m3 on 8th November. Since then, it has started to climb again and stood at 264 ug/m3 on 9th November. Hourly levels are again rising on 10th of November (checked up to 10 AM) and if weather conditions don’t improve, even 10th November might end up with a 24-hr average higher than 250 ug/m3

It should be noted that CSE assessment is based on monitoring in Delhi alone and daily average is considered from midnight to midnight, which is different from the CPCB’s methodology. The CPCB for its trend counter used for GRAP, uses an average of all stations in Delhi and NCR towns, while the CPCB AQI bulletin uses 4 PM to 4 PM as the measure of its daily value.  

Compared to the first smog episode of previous four years, current smog has matched the duration of first smog of 2018 and 2020 season -- both lasted six days. If conditions don’t improve, it might overtake the 2019 smog that lasted eight days. Average intensity of the smog this year so far was 329 ug/m3 per day, which is lower than that of the 2020 smog (by 7 per cent) and 2019 smog (by 3 per cent). It is more intense than the 2018 smog (about 9 per cent). Lingering on of this year’s smog for longer duration despite relatively windier local conditions might be due to the lack of pollution control measures in the city this year. 

Farm stubble fire smoke is lower this season: On an average, contribution of smoke to Delhi’s daily PM2.5 during middle of October to November 8th has been the lowest in the last four years. So far, it has recorded on an average 12 per cent per day in contrast to 17 per cent per day in 2020, 14 per cent per day in 2019, and 16 per cent per day in 2018 (as reported by SAFAR). If converted to absolute concentrations, the per day contribution of smoke stood at 26 ug/m3 this year so far compared to 35 ug/m3 in 2020, 40 ug/m3 in 2019, and 31 ug/m3 in 2018.

However, peak contribution of smoke to Delhi’s PM level was recorded on 7th November when it hit 48 per cent. This the second highest daily contribution percentage recorded since such estimation was started by SAFAR in 2018. The highest single day contribution percentage has been 58 per cent recorded on 5th November 2018. But this unusually high percentage on 7th November, 2021 didn’t spike Delhi’s PM2.5 levels as on that day PM2.5 levels came down to 265 ug/m3 from their 5th November 2021 peak of 501 ug/m3. 

Other toxic gases during smog episode: During this smog episode ozone levels have continued to breech standards at ozone hotspots in the city even during the peak smog episodes. Levels are even higher pre- and post-peak smog days. CO is also found to be exceeding the standard through the smog episode. SO2 levels mimic the trend of PM2.5 but do not breech the standard. NO2 bucks the trend and is seen to drop in concentrations when PM2.5 peaks. 

Delhi’s worst Diwali night in the last five years: PM2.5 concentration on Diwali night (8 PM to 8 AM) was the worst in the last five years. The 12-hr night average stood at 747 ug/m3, 22 per cent higher than that in 2020 Diwali night. The levels on Diwali night were 4.5 times the average night-time levels recorded in the week preceding Diwali. Please note that this Diwali night value is an undercount as the CPCB caps PM2.5 data at 1,000 ug/m3. It is noted that hourly concentrations can go beyond 1,000 ug/m3. This year, 26 of 38 operational monitoring stations hit the 1,000 ug/m3 mark. In 2020, 23 out of the 38 had hit the mark while in 2019 the number stood at 22 stations. Data on the DPCC website shows that hourly concentration went as high as 1,984 ug/m3 at Okhla Phase 2 and 1,957 ug/m3 at Ashok Vihar. The 12-hr night average for 24 DPCC stations stood at 824 ug/m3 using the uncapped data. This is 9 per cent higher than the average computed from the CPCB data for the same 24 stations. 

Diwali traffic shows up as elevated NO2 levels: Diwali night this year had lower NO2 levels compared to the 2020 Diwali night. The NO2 level in the last three years has been lower on Diwali night compared to nights preceding it, which is indicative of congestion and high traffic conditions in the city leading to the festival night – it is also reflective of less traffic on Diwali night itself.  DTU had the highest NO2 levels in the city with night-time average of 154 ug/m3. Dwarka Sector 8 and Anand Vihar with 107 ug/m3 and 103 ug/m3 respectively were the other NO2 hotspots on Diwali night. Rohini with just 2 ug/m3 of NO2 was the least affected area in the city followed by Nerala (5 ug/m3) and JNL Stadium (10 ug/m3). 

Act now

Says Roychowdhury: “The ongoing smog episode is a public health emergency. This requires urgent emergency action on key combustion sources (vehicles, industry, waste burning) and dust sources (construction and roads) to prevent further trapping of pollution when there is no wind to blow this away. We need pollution-source-wise and hotspot-wise status of action.” 

CSE has called for leveraging this to speed up longer term action for scaling up integrated public transport systems, walking and cycling; eliminate dirty fuels from all industrial units while tightening emissions control measures; and strengthen the municipal systems and infrastructure for segregated collection and recycling of waste. 

For more information, contact Sukanya Nair,, 8816818864.


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